Jockey William Buick is "conscious, but complaining of back pain" after he fell from the fatally-injured Permian in the Secretariat Stakes at Arlington Park, assistant trainer Charlie Johnston told Press Association Sport.
The Mark Johnston-trained colt had to be put down when the three-year-old son of Teofilo sustained a broken leg in the mile-and-a-quarter Grade One at the Illinois circuit.
Permian unseated Buick soon after the partnership crossed the line in last place.
Buick was taken to Northwest Community Hospital for further treatment.
Charlie Johnston, who is also the Middleham handler's son, told Press Association Sport: "We've lost the horse. He suffered a catastrophic fracture of his left fore.
"I was with him on the track for 30 seconds. He was in the process of running a very disappointing race and I believe it was when he was winding down that it happened.
"I am with William and he is conscious, but complaining of back pain."
Permian was one of the big success stories of this year's Classic generation, having won the Dante Stakes at York and the King Edward VII Stakes at Royal Ascot.
He earned over £400,000 in prize-money and won six of his 13 career starts.
A tweet posted by Arlington Park racecourse read: "William Buick was sitting up, talking & moving. Transported for evaluation & is reportedly in good spirits. Permian was humanely euthanized."